This invention relates to novel compositions and methods for thermally stabilizing O-pyridyl phosphate insecticides during its incorporation into thermoplastic resins.
Thermoplastic resins, polymers which can be softened and shaped by the application of heat and pressure and which return to their original condition when cooled to room temperature, are used in making articles such as underground pipelines and cables.
O-pyridyl phosphate insecticides have been incorporated into pipes and cables made from thermoplastic resins to make the resulting pipe or cable resistant to insect damage, particularly from ants and termites. Generally, in a known method for making insecticide-impregnated pipe, a thermoplastic resin, a stabilizer and a phthalate ester are kneaded together and then an O-pyridyl phosphate insecticide is incorporated therein. This resin containing the O-pyridyl phosphate insecticide is pelleted into beads or pellets which are later used to make the pipe.
However, a problem exists during the step of incorporation of the O-pyridyl phosphate insecticide into the resins as the O-pyridyl phosphates are thermally labile and unstable at the temperatures used for incorporation. At the temperatures of about 160.degree. to about 200.degree. C. used in the manufacture of plastic articles, the 0-pyridyl phosphates decompose, thereby preventing manufacturers from accurately and precisely metering the desired quantity of insecticide into the thermoplastic resin. The present invention overcomes this problem by providing a composition and a method for thermally stabilizing 0-pyridyl phosphate compounds during its incorporation into thermoplastic resins.